Relative Dating

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  • Created by: Jelena
  • Created on: 29-04-13 15:29

Relative Dating

  • Develops a sequence of events by comparing one rock layer to another --> doesn’t specify length of time between events

  • 4 principles used to determine relative dating

    1. The principle of superposition

    2. The principle of faunal succession

    3. The principle of crosscutting relations

    4. The principle of inclusion


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1. The principle of superposition

1. The principle of superposition

  • In a sequence of undeformed rock layers, the oldest rock beds are on the bottom and higher layers are successively younger

  • This principle assumes that layers were horizontal when deposited and the beds are still their relative position
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2. The principle of faunal succession

2. The principle of faunal succession

  • Based on observation that different rock layers contain a particular type of fossilized flora (plants) and fauna (animals) called index fossils

  • These fossil forms succeed each other in a specific and predictable order that can be identified over wide distances


Index fossils!

  • Certain fossils are found only in rock layers of a particular geological time = index fossils

  • To be an index fossil, they must meet some requirements:

    • Must be present in rocks scattered over a wide area of the Earth’s surface

    • Must have features that clearly distinguish them from all other fossils

    • The organisms from which the fossils formed must have lived during a relatively short geological time

  • Must occur in fairly large numbers within the rock layers
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3. The principle of crosscutting relations

3. The principle of crosscutting relations

  • An intrusion of igneous rock (dike) or a fault (a break in the rocks that make up Earth’s crust, along which rocks on either side have moved past each other) is younger than the rock it cuts across
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4. The principle of inclusion

4. The principle of inclusion

  • Relative age can also be determined where an overlaying rock layer contains particles of rock material from the layer beneath it

    • The bottom layer was eroded and the loose material on the surface became incorporated into the newly deposited layer

    • These particles, called inclusions, indicate the rocks in the lower layer are older than those on top
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